1. Field of the Invention
This invention relates to scaled driver devices that are provided to interface external devices operating at elevated voltage levels, and more particularly to a thin oxide output driver including scaled P-channel devices that are not subject to oxide breakdown at elevated voltages, and scaled N-channel devices that are not subject to hot carrier injection effects.
2. Description of the Related Art
As integrated circuit design and fabrication techniques have evolved over the years, the trend is that operating voltages have scaled downward along with device size. Very Large Scale Integrated (VLSI) circuits, particularly microprocessors, tend to lead in the area of size and voltage scaling. As a result, VLSI devices which operate at lower voltages are required to interface to external devices, such as input/output (I/O) devices or the like, that have not been scaled to the same extent as the VLSI devices. Nonetheless, the external devices must be driven to elevated voltage levels much higher than those of the VLSI device cores. As a result, many existing scaled VLSI devices provide voltage conversion circuits to increase the voltage swing of I/O signals so that they can properly interface to the elevated voltage external devices.
In more recent years, VLSI device sizes and operating voltages have decreased to the extent that, in some cases, scaled P-channel devices that provide an interface to elevated voltage external devices experience gate oxide breakdown if those same elevated voltage levels are used to drive their inputs. Because these P-channel devices have been significantly scaled, their gate oxide thickness is so thin that, if their gate is taken to the lowest voltage in the digital voltage range (e.g., 0 volts) while their source is tied to the elevated voltage (e.g., 3.3 volts), then the source-to-gate voltage VSG, the channel-to-gate voltage VCG, and the drain-to-gate voltage VDG, all exceed the breakdown voltage of the gate oxide, referred to as VBROX.
For example, VLSI devices today are fabricated using a 0.18 micron process that results in a gate oxide thickness of approximately 40 angstroms (Å) on a typical device. Skilled artisans will appreciate that the breakdown voltage for silicon dioxide (SiO2) is roughly 107 volts per centimeter (V/cm), and they also appreciate that it is prudent to restrict gate voltages to approximately 60 percent of the breakdown value. Hence, a prudent breakdown threshold, VBROX, for a 0.18 micron device is approximately 2.4 volts. The 0.18 micron devices are typically operated at VDD=1.8 volts referenced to ground at 0 volts, so that they generate a logic one (1) at 1.8 volts and a logic 0 at 0 volts. Thus, gate oxide breakdown at core voltage levels is not a problem.
The scaled driver devices of a VLSI device are typically required to interface to external Complementary Metal-Oxide Semiconductor (CMOS) devices that operate at higher voltage levels, such as 3.3 volts. As a result, pulling a 0.18 micron P-channel output device up to 3.3 volts, while holding its gate at zero volts will very likely damage the gate oxide of the P-channel device. Output voltage scaling circuits are known that operate to shift a logic 1 at a core voltage level up to the elevated level of the external devices, and to shift a logic 0 from 0 volts up to an intermediate voltage level. The intermediate voltage level is chosen low enough to turn on a P-channel device, yet high enough to avoid breakdown of the gate oxide.
Notwithstanding the protection afforded by output voltage scaling circuits, a conventional scaled output driver circuit experiences related problems when driving elevated voltages on a tri-state bus, such as when the P-channel device is turned off and the bus is pulled low. As further described herein, a portion of the gate oxide that overlaps the drain P-type diffusion area is exposed to excessive voltage, thus causing the oxide in the overlap area to break down.
Another problem occurs in the N-channel portion of the scaled driver device due to hot carrier injection effects. Hot carrier injection effects occur in N-channel device with short channels and thin gate oxide. Under repetitive switching of elevated voltages, the carriers accelerate to the extent that they get trapped in the oxide. This trapped charge can shift the threshold of the device and degrade its performance over time. Although hot carrier effects may be precluded in scaled N-channel devices by lowering the supply voltage, such solution is inapplicable in cases in which the scaled driver devices are required to interface elevated voltages, since the supply voltage cannot be lowered.
Therefore, what is needed is to provide a scaled driver device which is protected from gate oxide breakdown when turned off. It is also desired to protect the scaled driver device from hot carrier injection effects caused by repetitive switching of elevated voltages so that their performance is not degraded over time.